"My only love, sprung from my only hate!" (I.v.136) Meaning our families are in a vendetta.

"Yet, if thou swear'st/Thou love mayst prove false." (II.ii.91-92) Meaning, Romeo is swearing that he loves her much too fast.

"Swear not by the moon.../Lest that thy love prove likewise variable." (II.ii.109-111) Meaning that Romeo wants to swear by the moon, a fickle object. His love may prove fickle as well.

"I have no joy of this contract tonight./It is too rash, too unadvis'd, too sudden;" (II.ii.118-119) Meaning exactly what your assignment is asking: Juliet has sworn her love much too quickly and regrets that she has done so.